1. Field of the Invention
Illustrative embodiments of the present invention relate to a fixing device and an image forming apparatus, such as a copier and a laser printer, employing the fixing device.
2. Description of the Background
An image forming apparatus is used as a printer, a facsimile machine, a copier, a plotter, or a multi-functional peripheral having several of the foregoing capabilities. One conventional image forming apparatus performs image formation by an electrographic method. Such an electrophotographic image forming apparatus includes a fixing device that fixes a toner image on a recording medium, such as a paper sheet, by heating and pressing the toner image onto the sheet.
As illustrated in FIG. 1, for example, one conventional fixing device includes a fixing roller 100 serving as a rotary fixing member including a heater 300 and a pressure roller 200 serving as a rotary pressure member contacting the fixing roller 100 with pressure. When a recording medium P passes between the fixing roller 100 and the pressure roller 200 at a contact portion thereof, the recording medium P is heated and pressed so that an unfixed toner image T is fixed on the recording medium P.
After the heater 300 is turned on, the fixing device performs a warm-up operation to raise the temperature of the fixing roller 100 to a fixing target temperature at which the unfixed toner image T can be fixed on the recording medium P. When the heater 300 is turned on in the warm-up operation with the fixing roller 100 and the pressure roller 200 stopped, the temperature of the fixing roller 100 rises across the entire surface of the fixing roller but the temperature of the pressure roller 200 rises only in an area surrounding that portion of the pressure roller 200 contacting the fixing roller 100. In such a case, even if the fixing roller 100 rises to the target temperature, when the fixing roller 100 and the pressure roller 200 are rotated for fixing operation, the heat of the fixing roller 100 is absorbed by the unevenly heated pressure roller 200, causing fixing failure.
By contrast, when heating is performed while rotating the fixing roller 100 and the pressure roller 200, the temperature of both the fixing roller 100 and the pressure roller 200 as a whole rises, preventing such fixing failure. However, in the warm-up operation, heat is radiated from the pressure roller 200, thus lengthening warm-up time.
In such a situation, certain approaches have been proposed to shorten the warm-up time.
For example, one conventional fixing device stops rotating a fixing roller from the turning-on of a heater until the temperature of the fixing roller rises to a first setting temperature, and starts rotating the fixing roller when the temperature of the fixing roller has reached the first setting temperature. Such a configuration may prevent heat radiation from a heating roller, enhancing the speed of temperature rise.
Another conventional fixing device stops rotating a fixing roller from the turning-on of a heater until the temperature of the fixing roller rises to a fixing target temperature. Such a configuration may prevent heat radiation until the temperature of the fixing roller rises to the target temperature, thus enhancing the speed of temperature rise. After the temperature of the fixing roller has reached the target temperature, the rotation of the fixing roller is started to raise the temperature of a pressure roller.
As described above, in the above-mentioned conventional fixing devices, by stopping rotation of the fixing roller and the pressure roller in the warm-up operation, warm-up time is shortened. However, in such cases, since the fixing roller and the pressure roller contact each other, the heat of the heater is conducted to the pressure roller via the fixing roller and is radiated from the pressure roller. Such heat radiation from the pressure roller lengthens warm-up time.